Waterjet propulsion systems comprise one or more waterjets that increase the speed of water as it passes through the system. The power to drive the waterjet pump is typically provided by a gasoline or diesel engine, or in some cases, by a gas turbine, electric motor, or hydraulic motor. Water from an intake system in the vessel is fed through an inlet assembly and into an inboard impeller which adds energy to the water. From the impeller, the water flows through a stator which removes water swirl generated by the impeller, effectively straightening the flow. The water is then discharged at the stern through a stator assembly nozzle as a straight, high speed jet producing acceleration that generates thrust which propels the vessel.
Waterjets can be used for nearly any vessel or marine application including but not limited to ferries, high-speed watercraft, shallow water operations, and yachts. One application for waterjets includes amphibious watercraft such as ship-to-shore troop transporters or Amphibious Assault Vehicles (“AAV”). An AAV is an armed and armored military vessel with a boat-like hull designed to deliver assault troops and their equipment from ship to shore during military operations. A typical AAV weighs approximately 25 tons and has top speeds of about 8 knots in water.
During land use, a typical AAV employs a power driven continuous track that grips the road and propels the vessel. The marine propulsion system must be designed around these tracks and the armored exterior.
A watercraft propulsion system that utilizes waterjets is better suited for AAV applications when compared to a conventional screw propeller. Waterjet propulsion systems offer better maneuverability at low speeds, better operating ability in shallow waters, a higher debris tolerance because the rotating parts are shielded, and high thrust capabilities at the low speeds typical of AAVs.
Despite the obvious advantages over conventional screw propellers, waterjets are not without their drawbacks and AAV application of waterjets still present design challenges. AAVs are designed to carry crew and must also house weapons and other supplies in addition to its land propulsion system. Thus physical space for the water propulsion system is not readily available on an AAV. Additionally, any added volume can impact the hydrodynamics of the vessel. Although these issues are not entirely unique to AAVs, AAVs are particularly illustrative of the issue as they have a heavily restricted intake area due to the tracks, armor, and hull of the vessel and thus require a high power density propulsion system to conserve space and volume.
Many AAVs require a waterjet to be retrofittable to the existing layout and drive shaft. This creates problems with matching torque, horsepower, and RPM requirements of the AAV with volume and design criteria of the waterjet. Additionally, because AAVs operate not only as a watercraft, but also on land, the waterjets are routinely exposed to jolts and vibrations from land travel that are unique to AAVs as compared to other marine vessels. Sea based propulsion systems are not designed to bear the stress caused by prolonged land transportation, particularly the off-road environments in which AAVs are commonly used. Vibrations from land travel act as compounded wear and tear for a sea based propulsion system and components. Thus, such actions can cause serious fabrication and weld failure of a waterjet propulsion system that was designed to bear sea loads.
AAVs are generally used for military operations; any failure in the waterjet propulsion system can be catastrophic to the human personnel on board and mission success. Thus, it is necessary that the system be robust to withstand military operation conditions. Additionally, routine maintenance of the system requires an easy access and removal process to ensure that the military operation is not disrupted. Currently, no waterjet application exists that is capable of consistently withstanding the stress caused by both land and sea travel, while allowing easy access for maintenance and replacement.
For the foregoing reasons, there is a need for a waterjet propulsion system in AAV applications that, in one or more embodiments, is capable of being retrofitted and customized, can withstand vibrational fatigue, and can be easily removed or accessed for maintenance, repair, or replacement, while providing the necessary thrust and propulsion for the applicable task.